Devil May Cry walkthrough/SM01

To get to this mission, you have to first kill Phantom, then go back to the Broken Bridge area and deliberately jump down. That will activate the Secret Mission.

You have to kill the Sin Scissors with one blow. Easiest way to do this is to use the Shotgun at range to stun it: The blast should hit it in the scissors and only knock it off balance, and not damage it. When it's stunned, run in to point-blank range and blast it again in the face. This should do it. Devil Trigger might help if you can't get it the first time around.